I think it depends on what you’re walking to. Like the previous poster, I like walking – but for a place to be considered truly “walkable” (and for me to consider living there since walkability is a priority for me) I judge it on whether I’d walk the distance in just about any weather. That obviously means that safety has to be accounted for. Other than that, judging from this cold snap we’ve been having I’d be unlikely to take the Metro if it was more than 10 minutes walking – especially for anything other than getting to work. For buses it’s more like 5-7 minutes (and then I would only consider the area “walkable” if it is on bus lines with very frequent service – the 30’s, S’s, 50’s, Circulator, etc.). For things like groceries or nightlife, 15 minutes is about as far as I’ll walk before considering alternative ways to get there (biking, bus, cab/car).